This invention relates to a control circuit utilizing silicon controlled rectifiers (SCR's) for controlling the operation of a series motor powered from a direct-current source.
It is well known that the direct current supplied to a direct-current motor from a power source of constant potential, such as a battery, may be selectively varied by controlling the average power to the motor, and that a solid state SCR can be used as a switching device to repeatedly connect and disconnect the battery to and from the motor. The power supplied to the motor is determined by the ratio between the time the SCR is turned on and is conducting and the time the SCR is turned off and is nonconducting.
Turning the SCR repeatedly on and off will allow a series of pulses of current to flow through the SCR and the motor, the frequency of pulses being determined by the number of times the SCR is turned on per unit time and the duration or pulse width being determined by the length of time that the SCR remains on before it is turned off. If the pulse width remains constant, the ratio of on-time to off-time will vary directly as the pulse frequency. As the pulse frequency increases, the off-time (between the time the SCR is turned off and the time it is turned back on) will decrease, and more power will be delivered to the load.
The average power delivered to the load may also be varied by maintaining the same pulse frequency and varying the pulse width. A longer pulse width will cause more power to be delivered to the motor than will a shorter pulse width.
Typically, SCR control systems include a main SCR which is connected in series with the motor and the battery and a pulse generator which repeatedly supplies gate pulses to turn the main SCR on. A commutating capacitor is provided which will charge in a commutating direction through a charging SCR. The charging is typically done when the main SCR is conducting. At the appropriate time, a commutating SCR is turned on to connect the charged capacitor across the main SCR and divert current therefrom so that the main SCR is reversely biased and will turn off.
When a vehicle is traveling under power in one direction, braking may be accomplished by reversing the connection of the field to the armature. With the direction of current flow through the field being reversed, the motor will act as a generator, generating current flow (commonly referred to as plug current) through the armature which provides dynamic braking of the vehicle.
When the motor is plugging it is desirable to discontinue operation of the main SCR so that no power is delivered therethrough to the motor until the plug current decays to a safe level. Even then, when the main SCR is reactivated to supply power therethrough to the plugging motor it is desirable to supply only a small amount of power therethrough so that the vehicle can be brought to a smooth and controlled halt before the direction of the vehicle is reversed. Once the vehicle has come to a halt, normal operation of the main SCR should be resumed so that the vehicle will accelerate in the reverse direction.
A problem which exists in vehicles having present SCR controls is that plug current is sensed and used to control the resumption of normal SCR operation to move the vehicle in the opposite direction when the plug current has ceased. Very often the plug current will decay to such a low level that the plug current sensor will consider plug current to have ceased and will cause the SCR control to apply power to the motor for opposite motion even though the vehicle had not yet come to rest. This will cause a harsh, abrupt stop at the end of a relatively smooth plugging stop. This has been found to be especially undesirable in the operation of loaded fork-lift trucks, plugging from forward direction, since the abrupt stop causes forward tilting of the load and causes the rear steering wheels to lift up and lose steering contact with the ground.